1. Field of the Invention
The invention relates to a lithographic projection apparatus.
2. Brief Description of Related Art
In a lithographic projection apparatus in general, it is of paramount importance to be able to detect whether or not an object has been placed correctly on a support structure. This may for instance be the case for a wafer on a wafer table or for a mask on a mask table. To this end the support structure and/or the object are designed such that together, i.e. with an object placed on the support structure they define a closed space. In a lithographic apparatus working under atmospheric conditions an object is normally connected to a support structure, or “clamped”, by evacuating the closed space. The presence of an object on the support structure is then detected by measuring the pressure in the space or flow from the space. When an object is present, the pressure or flow is different from the case when no object is present. This principle is, however, not feasible for a lithographic apparatus working under relatively high vacuum conditions such as lithographic apparatus using extreme ultraviolet radiation (EUV).
In such a vacuum environment clamping is effected by electrostatic forces. Capacitive measurements have been proposed in order to detect whether an object has been placed correctly or has good contact. Such measurements are also not very well feasible, because of the sensitivity to electromagnetic disturbances, or due to the large variety of object materials each having different electrical properties, causing different changes in capacitances.
From the above, it is clear that a problem in the present state of the art is to assess, under vacuum conditions, whether or not an object has been placed correctly on a support structure, like e.g. a wafer on a wafer table, a so called “handler clamp” or an “exposure pin”, or a mask on a chuck.
A solution for the problem is for example proposed in the U.S. Pat. No. 6,576,483 B1 in which a system for supporting and clamping a semiconductor wafer is disclosed. The known system comprises an electrode and a clamp between which the wafer can be clamped. The electrode comprises a cannelure open towards the backside of the wafer, whereby the cannelure exposes the backside to a gas piped through one or more holes provided in the electrode. The holes are connected in turn to a gas supply structure. The cannelure of the known system has a size such that deviation of the wafer from its proper position by more than a threshold partially exposes the cannelure, such that the gas leaks from the cannelure. The supply structure comprises a gas flow detector for detecting gas leaking from the cannelure and provides corresponding detection of the wafer deviating from its proper position.
A similar solution is proposed in U.S. Pat. No. 6,401,359 B1, which discloses a system capable of detecting leakage of a heat-transfer gas supplied to a space between a wafer and a mounting stand supporting the wafer in order to determine whether or not the wafer is correctly placed on the mounting stand. The system disclosed in U.S. Pat. No. 6,401,359 B1 uses a flowmeter to perform the detection. The flowmeter measures the flowrate of the gas and compares it by a comparator against a threshold value.
A drawback of the known systems is that when used in a lithographic apparatus using high vacuum, the actual detection of a misplaced wafer or mask may take to long, i.e. an unacceptable amount of gas may be released into the vacuum space surrounding the object.